The objective of this research project is to understand the molecular mechanisms responsible for the condensation of the mucin polymer network inside secretory granules; its expansion upon release from the cell; and its final equilibrium swelling to form the mucus gel. The approach is to use the established knowledge of polymer gel physics that has been learned from artificial polymer gels to verify if the mucus gel follows the same molecular mechanisms as other polymer gels and get a hold of the fundamental physical laws that govern the storage, release and final equilibrium swelling of mucins. We have successfully used this approach during the last ten years. We have developed the appropriate experimental models and methods to investigate the molecular conformation and topology of the mucus gel; we have established that the swelling in mucus is a critical mechanism to control mucus rheology and that like in other polyionic gels, mucus swelling is governed by a Donnan equilibrium process. Predictions based on polymer gel physics allowed us to confirm the existence and define the role of calcium as the cations responsible for charge shielding in condensed presecretory mucins. Preliminary experiments from our laboratory show that mucins, like other gels, can undergo a typical polymer gel phase transition that could eventually explain the physical mechanisms of condensation and expansion of mucin during storage and releases in and from secretory cells. Support from this grant will be used to further investigate the role of ions and polyions in the swelling of mucins upon release from goblet cells in culture; to further identify the cations responsible for the shielding of mucin charges during storage inside secretory granules; and to further verify and characterize the novel finding of a polymer gel phase transition that we have recently discovered in mucin gels.